Streamlined methane gas generation system

ABSTRACT

A process for producing high purity methane gas from digested or composted organic materials as well as a high purity methane gas generation system that operates in accordance with this process are provided. The inventive system, which is capable of delivering at least about 0.5 slpm of methane product gas, is streamlined in design and provides a more reliable and cost-effective source of renewable methane gas.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/766,552, filed Jan. 27, 2004, which claims priority fromU.S. Provisional Patent Application Ser. No. 60/443,410, filed Jan. 29,2003.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to a process for producing highpurity methane gas from biogas or a mixed gas stream and further relatesto a streamlined high purity methane gas generation system that operatesin accordance with the inventive process.

BACKGROUND ART

High purity methane gas production by digestion or composting of organicmaterials offers promise for solving alternative energy problems. Forexample, in agricultural settings this technology could be used tooffset energy expenses, to control odor, and to produce a marketableproduct.

Digestion occurs when bacteria produce biogas by decomposing organicmatter in the presence of air for aerobic digestion, or in anenvironment that is devoid of air for anaerobic digestion. Interest inon-farm biogas generation was prompted by the energy crisis of the1970s, with several farm operations throughout the country experimentingwith anaerobic digesters. This experimentation, however, was frequentlymarked by dissatisfaction on the part of farm owners due to high systemcost and/or system failure. The lack of economic feasibility andtechnological failure appear responsible for the fact that anaerobicdigestion has not been widely used in agricultural settings.

Thus, a need exists for a more reliable and cost effective system forgenerating renewable sources of high purity methane gas.

It is therefore an object of the present invention to provide such asystem.

It is a more particular object to provide a high purity methane gasgeneration system that is streamlined in design and-that reliably andcost-effectively converts organic materials such as animal waste intothe desired methane product gas.

It is another more particular object to provide a reliable,cost-effective and streamlined high purity methane gas generation systemthat is self-sustaining during normal operation, requiring no outsidesources of energy (e.g., electric, gas).

SUMMARY

The present invention therefore provides a process for producing highpurity methane gas from digested or composted organic materials, whichcomprises:

-   -   1) digesting or composting organic materials to produce a biogas        or mixed gas stream containing methane, carbon dioxide and trace        impurities such as hydrogen sulfide, nitrogen and oxygen,        wherein the mixed gas stream may also contain small quantities        of water;    -   2) directing the biogas or mixed gas stream to a first mixed gas        purification device for removing at least a portion of the        hydrogen sulfide from the mixed gas stream;    -   3) directing portions of the desulfurized, mixed gas stream to        one or more mixed gas-driven engines that power one or more        compressors, wherein the mixed gas stream serves to fuel the one        or more mixed gas-driven engines;    -   4) directing remaining portions of the desulfurized, mixed gas        stream to the one or more compressors for compressing the mixed        gas stream; and    -   5) directing the compressed, desulfurized, mixed gas stream to a        second mixed gas purification device for removing at least a        portion of the carbon dioxide and any excess nitrogen from the        mixed gas stream to produce a high purity methane gas product        stream.

The present invention further provides a streamlined, high puritymethane gas generation system. The inventive system uses biogas to fuelone or more mixed gas-driven compressors employed therein, and basicallycomprises:

-   -   1) one or more devices for digesting and/or composting organic        materials to produce a biogas or mixed gas stream containing        methane, carbon dioxide and trace impurities such as hydrogen        sulfide, nitrogen and oxygen;    -   2) a first mixed gas purification device, which serves to remove        at least a portion of the hydrogen sulfide from the mixed gas        stream, wherein the first mixed gas purification device is in        fluid communication with the one or more devices for digesting        and/or composting organic materials;    -   3) at least one mixed gas-driven compressor, which serves to        compress or pressurize the desulfurized, mixed gas stream,        wherein the at least one mixed gas-driven compressor is in fluid        communication with the first mixed gas purification device; and    -   4) a second mixed gas purification device, which serves to        remove at least a portion of the carbon dioxide and any excess        nitrogen from the compressed, desulfurized, mixed gas stream to        produce a high purity methane gas product stream, wherein the        second mixed gas purification device is in fluid communication        with the at least one mixed gas-driven compressor.

Other features and advantages of the invention will be apparent to oneof ordinary skill from the following detailed description andaccompanying drawings.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. All publications, patentapplications, patents and other references mentioned herein areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and notintended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular features of the disclosed invention are illustrated byreference to the accompanying drawings in which:

FIG. 1 is a process flow schematic of a preferred embodiment of the highpurity methane gas generation system of the present invention; and

FIG. 2 is a process flow schematic of a more preferred embodiment of thehigh purity methane gas generation system of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The methane gas generation system of the present invention is capable ofdelivering at least one (1) cubic foot per hour at standard conditions(1 scfh) or 0.5 standard liters per minute (0.5 slpm) of the desiredmethane gas product stream. The inventive system is streamlined indesign and provides a more reliable and cost-effective source ofrenewable methane gas. Further, the inventive system depends upondigested or composted organic materials and not fossil fuels to producehigh purity methane gas. Moreover, in a preferred embodiment, theinvention system is self-sustaining during normal operation, requiringno outside sources of energy (e.g., electric, natural gas).

Referring now to FIG. 1 in detail, a process flow schematic of apreferred embodiment of the high purity methane gas generation system ofthe present invention is shown and generally designated by referencenumeral 10. In this embodiment, organic materials such as plantmaterial, animal waste, food waste, or human waste, are fed into ananaerobic or aerobic digester or composter 12. For anaerobic digesters,an anaerobic microbe would be added to digester 12 to form a reactionsolution. The digestion period would be allowed to last for from about 3to about 21 days, while forming a biogas or mixed gas stream containingpredominantly methane and carbon dioxide along with trace impuritiessuch as hydrogen sulfide, nitrogen and oxygen. As will be readilyapparent to those skilled in the art, the length of the anaerobicdigestion period depends upon the type of organic materials beingprocessed and the design of the digester.

The biogas or mixed gas stream emanating from digester or composter 12is passed to a first mixed gas purification device 14 for removing atleast a portion of the hydrogen sulfide from the stream to maximize thelife of, for example, downstream molecular sieves or adsorbents. As iswell known to those skilled in the art, hydrogen sulfide may be removedusing conventional hydrogen sulfide scrubbers, absorbents or adsorbentssuch as activated charcoal and bituminous coal, zinc oxide, and mixturesthereof.

In a preferred embodiment, hydrogen sulfide is removed by catalyticcarbon. More specifically, activated bituminous coal is employed in thefirst mixed gas purification device 14 and a small amount of air isadded to the stream prior to entering device 14 so as to facilitate thefollowing reaction on the activated bituminous coal:2H₂S+O₂→2 H₂O+2S

In a more preferred embodiment, the first mixed gas purification device14 is a carbon adsorber, which is available from USFilter WestatesCarbon, 10 Technology Drive, Lowell, Mass. 01851, under the productdesignation VENT-SCRUB™ VSC-Series VSC1000 carbon adsorber, preloadedwith Midas OCM odor control media.

A portion of the desulfurized, mixed gas stream is then passed to atleast one mixed gas-driven engine 16, which powers at least onecompressor 18, while remaining portions are passed to the compressor 18,which compresses the gas to a pressure exceeding the operating pressureof a second mixed gas purification device 20. In a preferred embodiment,the gas is compressed to a pressure of up to about 1.18 megapascals(MPa), more preferably up to about 1.38 MPa.

A portion of the mixed gas stream (either before or after hydrogensulfide removal) may also be passed to one or more mixed gas-drivenengines (not shown) for powering one or more generators (also notshown). The generator(s) would produce electric energy for on site useto offset energy expenses and/or for export to electricity grids. One ormore batteries may be used in conjunction with the generator(s).

Upon leaving compressor 18, the compressed, desulfurized, mixed gasstream is directed to the second mixed gas purification device 20 forremoving at least a portion of the carbon dioxide and any excessnitrogen (i.e., nitrogen levels exceeding 0.5% by volume in the mixedgas stream) contained therein.

The second mixed gas purification device 20 is not limited. In apreferred embodiment, device 20 is a membrane-based separation device orsystem that employs at least one membrane having carbon dioxideselectivity and optionally, also employs at least one membrane havingnitrogen selectivity. In a more preferred embodiment, the second mixedgas purification device 20 is a pressure swing absorption (PSA) deviceor system comprised of at least two molecular sieve chambers. The PSAdevice or system may be used alone or in combination with themembrane-based separation device or system.

Generally speaking, in the contemplated PSA device or system, the mixedgas stream would be passed to at least one of a plurality of adsorptionzones at an elevated pressure effective to adsorb carbon dioxide and anyexcess nitrogen (i.e., the more strongly adsorbed components), while atleast methane would pass through (i.e., the less strongly adsorbedcomponent(s)). At a defined time, the passing of the mixed gas stream tothe PSA device or system would be terminated and the adsorption zone(s)would be depressurized by one or more concurrent depressurization stepswhere the pressure would be reduced to a defined level which wouldpermit the separated, less strongly adsorbed methane remaining in theadsorption zone(s) to be drawn off. Then, the adsorption zone(s) wouldbe depressurized by a counter-current depressurization step where thepressure in the adsorption zone(s) would be further reduced bywithdrawing desorbed gas counter-currently to the direction of the mixedgas stream. Finally, the adsorption zone(s) would be purged andre-pressurized. As is well known to those skilled in the art, the PSAprocess is generally carried out in a sequential processing cycle thatincludes each bed of the PSA device or system.

In a more preferred embodiment, the PSA device or system 20 is comprisedof a housing and at least two molecular sieve chambers (preferably, fromabout 5 to about 10 molecular sieve chambers) contained within thehousing for receiving a molecular sieve or adsorbent for separatingcarbon dioxide and any excess nitrogen from the mixed gas stream.

Molecular sieves or adsorbents suitable for use in the present inventionare not limited and include carbon fiber composite molecular sieves,zeolite molecular sieves, as well as, other molecularly selective media.

In yet a more preferred embodiment, the second mixed gas purificationdevice 20 is a rotary valve driven nine bed PSA device or system, whichis available from QuestAir Technologies Inc., 6961 Russell Avenue,Burnaby, BC V5J 4R8, under the product designation QuestAir M-3200 PSAgas separator.

The process for producing high purity methane gas embodied within system10, as shown in FIG. 1, may be summarized as set forth below:

-   -   1) digesting or composting organic materials to produce a biogas        or mixed gas stream containing methane, carbon dioxide and trace        impurities such as hydrogen sulfide, nitrogen and oxygen,        wherein the mixed gas stream may also contain small quantities        of water;    -   2) directing the biogas or mixed gas stream to a first mixed gas        purification device 14 for removing at least a portion of the        hydrogen sulfide from the mixed gas stream;    -   3) directing portions of the desulfurized, mixed gas stream to        one or more mixed gas-driven engines 16 that power one or more        compressors 18, wherein the mixed gas stream serves to fuel the        one or more mixed gas-driven engines 16;    -   4) directing remaining portions of the desulfurized, mixed gas        stream to the one or more compressors 18 for compressing the        mixed gas stream; and    -   5) directing the compressed, desulfurized, mixed gas stream to a        second mixed gas purification device 20 for removing at least a        portion of the carbon dioxide and any excess nitrogen from the        mixed gas stream to produce a high purity methane gas product        stream.

The inventive system 10 produces up to about 3000 slpm of methane gasat >90% purity, at pressures ranging from about 1.0 to about 1.4 MPa.The methane gas product stream exiting the second mixed gas purificationdevice 20 meets the Society of Automotive Engineers (SAE) standards forcompressed natural gas (CNG) and may be used for low-pressureapplications such as fuel for natural gas engines and any other devicesthat use pipeline natural gas.

In a more preferred embodiment and as best shown in FIG. 2, the methanegeneration system 10 of the present invention, may further comprise: (1)means 22 for removing at least a portion of any water contained in themixed gas stream, which is in fluid communication with the digester orcomposter 12; (2) means 24 for introducing air into the mixed gas streamto facilitate hydrogen sulfide removal in the first mixed gaspurification device 14, which is in fluid communication with thedigester or composter 12; (3) means 26 for heating the mixed gas streamto prevent water “drop out” and to facilitate hydrogen sulfide removalin the first mixed gas purification device 14, which is in fluidcommunication with the digester or composter 12; (4) optionally, means(not shown) for removing water from the mixed gas stream, which is influid communication with the second mixed gas purification device 20;(5) means 28 for neutralizing sudden pressure surges in the methane gasproduct stream, which is in fluid communication with the second mixedgas purification device 20; and (6) optionally, one or more highpressure, mixed-gas driven compressors for further compressing themethane gas product stream for higher-pressure applications (e.g.,pressures ranging from about 4.8 to about 25.0 MPa for adding theproduct stream to natural gas pipelines and/or for fueling CNGvehicles), which is/are in fluid communication with the second mixed gaspurification device 20.

Means 22 for removing at least a portion of any water contained in themixed gas stream is not limited and, in a preferred embodiment,comprises a coalescing filter in which small water droplets present inthe mixed gas stream combine to form larger droplets that are ofsufficient size to accumulate or collect in the filter housing. In amore preferred embodiment, coalescing filter 22 serves to reduce therelative humidity in the mixed gas stream to a level ranging from about70 to about 95%.

Means 24 for introducing air into the mixed gas stream to facilitatehydrogen sulfide removal in the first mixed gas purification device 14is also not limited and, in a preferred embodiment, comprises an airinjection system capable of providing air to the mixed gas stream in anamount ranging from about 10 to about 500 times stoichiometric. At lowflow conditions (i.e., 20% rated) the air injection system 24 providesno more than about 0.5% by volume, based on the total volume of themixed gas stream, of air to the mixed gas stream, while at high flowconditions (i.e., 100% rated) system 24 provides an amount of air to themixed gas stream equal to at least about 10 times the amount of totalsulfur in the gas stream.

Means 26 for heating the mixed gas stream to prevent water “drop out”and to facilitate hydrogen sulfide removal in the first mixed gaspurification device 14 is also not limited and, in a preferredembodiment, comprises a heating device capable of heating the mixed gasstream to a temperature ranging from about 15 to about 33° C. before themixed gas stream enters the first mixed gas purification device 14.

Means 28 for neutralizing sudden pressure surges in gas streams is knownand includes, but are not limited to, surge tanks capable of holding oneto five times the volume of high purity methane gas emanating from thesecond mixed gas purification device 20.

The process for producing high purity methane gas, which is embodiedwithin the more preferred embodiment of device 10, as shown in FIG. 2,may be summarized as set forth below:

-   -   (a) digesting or composting organic materials to produce a mixed        gas stream containing methane, carbon dioxide and trace        impurities including hydrogen sulfide, nitrogen and oxygen,        wherein the mixed gas stream may also contain small quantities        of water;    -   (b) directing the mixed gas stream to: (i) means 22 for removing        at least a portion of any water contained in the mixed gas        stream; (ii) means 24 for aerating the mixed gas stream;        and (iii) means 26 for heating the mixed gas stream to a        temperature ranging from about 15 to about 33° C.;    -   (c) directing the condensed, aerated and heated mixed gas stream        to a first mixed gas purification device 14 for removing at        least a portion of the hydrogen sulfide contained therein;    -   (d) directing a portion of the desulfurized, mixed gas stream to        a mixed gas-driven engine 16 for fueling same, wherein the mixed        gas-driven engine 16 powers two compressors 34, 36;    -   (e) directing remaining portions of the desulfurized, mixed gas        stream to the compressors 34, 36 for compressing the mixed gas        stream to a pressure of up to about 1.38 MPa;    -   (f) directing the compressed, desulfurized, mixed gas stream to        a second mixed gas purification device 20 for removing at least        a portion of the carbon dioxide and any excess nitrogen        contained therein to produce a high purity methane gas product        stream; and    -   (g) directing the high purity methane gas product stream to        means 28 for neutralizing sudden pressure surges in the product        stream.

The more preferred embodiment of inventive system 10 produces from about200 to about 1000 slpm of methane gas at >98.5% purity, at a pressure ofabout 1.4 MPa.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be apparent to those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the invention.

1. A streamlined process for producing high purity methane gas fromdigested or composted organic materials, which comprises: (a) digestingor composting organic materials to produce a mixed gas stream containingmethane, carbon dioxide and trace impurities including hydrogen sulfide,nitrogen and oxygen, wherein the mixed gas stream may also contain smallquantities of water; (b) directing the biogas or mixed gas stream to afirst mixed gas purification device for removing at least a portion ofthe hydrogen sulfide from the mixed gas stream; (c) directing portionsof the desulfurized, mixed gas stream to one or more mixed gas-drivenengines that power one or more compressors, wherein the mixed gas streamserves to fuel the one or more mixed gas-driven engines; (d) directingremaining portions of the desulfurized, mixed gas stream to the one ormore compressors for compressing the mixed gas stream; and (e) directingthe compressed, desulfurized, mixed gas stream to a second mixed gaspurification device for removing at least a portion of the carbondioxide and any excess nitrogen from the mixed gas stream to produce ahigh purity methane gas product stream.
 2. The streamlined process forproducing high purity methane gas of claim 1, wherein the biogas ormixed gas stream is produced by anaerobically digesting the organicmaterials.
 3. The streamlined process for producing high purity methanegas of claim 1, which further comprises directing portions of the mixedgas stream or desulfurized, mixed gas stream to one or more mixedgas-driven engines for powering one or more generators.
 4. A streamlinedprocess for producing high purity methane gas from digested or compostedorganic materials, which comprises: (a) digesting or composting organicmaterials to produce a mixed gas stream containing methane, carbondioxide and trace impurities including hydrogen sulfide, nitrogen andoxygen, wherein the mixed gas stream may also contain small quantitiesof water; (b) directing the mixed gas stream to means for: (i) removingat least a portion of any water contained in the mixed gas stream; (ii)aerating the mixed gas stream; and (iii) heating the mixed gas stream toa temperature ranging from about 15 to about 33° C.; (c) directing thecondensed, aerated and heated mixed gas stream to a first mixed gaspurification device for removing at least a portion of the hydrogensulfide contained therein; (d) directing a portion of the desulfurized,mixed gas stream to one or more mixed gas-driven engines for fuelingsame, wherein the one or more mixed gas-driven engines power one or morecompressors; (e) directing remaining portions of the desulfurized, mixedgas stream to the one or more compressors for compressing the mixed gasstream; and (f) directing the compressed, desulfurized, mixed gas streamto a second mixed gas purification device for removing at least aportion of the carbon dioxide and any excess nitrogen contained thereinto produce a high purity methane gas product stream.
 5. The streamlinedprocess for producing high purity methane gas of claim 4, wherein themixed gas stream is produced by anaerobically digesting the organicmaterials.
 6. The streamlined process for producing high purity methanegas of claim 4, which further comprises directing portions of the mixedgas stream or desulfurized, mixed gas stream to one or more mixedgas-driven engines for powering one or more generators.
 7. Thestreamlined process for producing high purity methane gas of claim 4,which further comprises directing the high purity methane gas productstream to means for neutralizing sudden pressure surges in the productstream.
 8. A streamlined, high purity methane gas generation system,which comprises: (a) one or more devices for digesting and/or compostingorganic materials to produce a mixed gas stream containing methane,carbon dioxide and trace impurities including hydrogen sulfide, nitrogenand oxygen; (b) a first mixed gas purification device, which serves toremove at least a portion of the hydrogen sulfide from the mixed gasstream, wherein the first mixed gas purification device is in fluidcommunication with the one or more devices for digesting and/orcomposting organic materials; (c) at least one mixed gas-drivencompressor, which serves to compress the desulfurized, mixed gas stream,wherein the at least one mixed gas-driven compressor is in fluidcommunication with the first mixed gas purification device; and (d) asecond mixed gas purification device, which serves to remove at least aportion of the carbon dioxide and any excess nitrogen from thecompressed, desulfurized, mixed gas stream to produce a high puritymethane gas product stream, wherein the second mixed gas purificationdevice is in fluid communication with the at least one mixed gas-drivencompressor.
 9. The streamlined, high purity methane gas generationsystem of claim 8, wherein the system is self-sustaining duringoperation, requiring no outside sources of energy.
 10. The streamlined,high purity methane gas generation system of claim 8, wherein the one ormore devices for digesting and/or composting organic materials comprisesone or more anaerobic digesters.
 11. The streamlined, high puritymethane gas generation system of claim 8, which further comprises one ormore generators which are powered by one or more mixed gas-drivenengines.
 12. The streamlined, high purity methane gas generation systemof claim 8, wherein the second mixed gas purification device is apressure swing absorption device.
 13. The streamlined, high puritymethane gas generation system of claim 12, wherein the pressure swingabsorption device comprises at least two molecular sieve chambers. 14.The streamlined, high purity methane gas generation system of claim 8,which further comprises one or more high pressure, mixed gas-drivencompressors for further compressing the methane gas product stream,wherein the one or more high pressure, mixed gas-driven compressors arein fluid communication with the second mixed gas purification device.15. The streamlined, high purity methane gas generation system of claim8, which produces up to 3,000 standard liters per minute of methane gasat pressures ranging from about 1.0 to about 1.4 megapascals, whereinthe methane gas has a purity level of greater than 90%.
 16. Astreamlined, high purity methane gas generation system, which comprises:(a) one or more devices for digesting and/or composting organicmaterials to produce a mixed gas stream containing methane, carbondioxide and trace impurities including hydrogen sulfide, nitrogen andoxygen; (b) means for removing at least a portion of any water containedin the mixed gas stream, which is in fluid communication with the one ormore devices for digesting and/or composting organic materials; (c)means for aerating the mixed gas stream, which is in fluid communicationwith the one or more devices for digesting and/or composting organicmaterials; (d) means for heating the mixed gas stream to a temperatureranging from about 15 to about 33° C., which is in fluid communicationwith the one or more devices for digesting and/or composting organicmaterials; (e) a first mixed gas purification device, which serves toremove at least a portion of the hydrogen sulfide from the mixed gasstream, wherein the first mixed gas purification device is in fluidcommunication with the one or more devices for digesting and/orcomposting organic materials; (f) at least one mixed gas-drivencompressor in fluid communication with the first mixed gas purificationdevice; and (g) a second mixed gas purification device, which serves toremove at least a portion of the carbon dioxide and any excess nitrogenfrom the mixed gas stream to produce a methane gas product stream,wherein the second mixed gas purification device is in fluidcommunication with the at least one mixed-gas driven compressor.
 17. Thestreamlined, high purity methane gas generation system of claim 16,wherein the system is self-sustaining during operation, requiring nooutside sources of energy.
 18. The streamlined, high purity methane gasgeneration system of claim 16, wherein the one or more devices fordigesting and/or composting organic materials comprises one or moreanaerobic digesters.
 19. The streamlined, high purity methane gasgeneration system of claim 16, which further comprises one or moregenerators which are powered by one or more mixed gas-driven engines.20. The streamlined, high purity methane gas generation system of claim16, wherein the second mixed gas purification device is a pressure swingabsorption device.
 21. The streamlined, high purity methane gasgeneration system of claim 20, wherein the pressure swing absorptiondevice comprises at least two molecular sieve chambers.
 22. Thestreamlined, high purity methane gas generation system of claim 16,which further comprises one or more high pressure, mixed-gas drivencompressors for further compressing the methane gas product stream,wherein the one or more high pressure, mixed gas-driven compressors arein fluid communication with the second mixed gas purification device.23. The streamlined, high purity methane gas generation system of claim16, which produces up to 3,000 standard liters per minute of methane gasat pressures ranging from about 1.0 to about 1.4 megapascals, whereinthe methane gas has a purity level of greater than 90%.